1. Field of the Invention
This invention relates to an inductance element arranged to be built n a printed circuit board and more particularly to an inductance element which is usable also as noise-radiation preventing means for preventing a high frequency noise generated by a semiconductor device mounted on a printed circuit board from radiating around the printed circuit board.
2. Description of the Related Art
The recent advancement of technology for the manufacture of semiconductors has come to permit the manufacture of large-capacity memories which are accessible at a high speed and high-precision A/D converters which are capable of operating at a high speed. As a result, it has rapidly become popular to change internal signal processing circuits of electronic apparatuses from the conventional analog circuits to digital circuits.
When a signal processing circuit is digitized, the necessary numbers of logic gates and transistors increase very much from the numbers required for an analog circuit. Besides, since the frequencies of signals to be processed also become higher, there arises a high frequency noise over a wide band due to the switching actions of many transistors and logic gates arranged within semiconductor devices which form the signal processing circuit. The noise propagates to a signal transmitting cable (an interfacing cable) through the connection pin of the semiconductor device and the wiring of an electronic circuit board. Then, the noise radiates around the cable, with the cable acting as an antenna, to hinder the actions of other electronic apparatuses by intruding into their circuits. It is also known that such a noise tends to affect an electromagnetic environment by hindering wireless communication around it. To prevent such a trouble, advanced countries such as the U.S.A., etc., have recently put in force some legal regulations against noises generated by electronic apparatuses. In compliance with the legal regulations, therefore, recent electronic apparatuses are provided with some electronic parts that prevent such a noise from radiating. These noise-radiation preventing parts are known by the name of EMI filters. These EMI filters include a part which consists of a combination of a coil, i.e., an inductance element, and a capacitor (LC filter); a part which is formed solely with an inductance element; and a part which is formed solely with a capacitor.
Referring to FIG. 6, known choke coils 61 and 62 which have two rows of insertion pins corresponding to a DIP (dual in-line package) type semiconductor device are arranged to be mounted on a lead-through (lead-or pin-inserting) mount type circuit board 64. The choke coil can be used not only as an ordinary circuit element but also as a noise-radiation preventing part. FIG. 6 further shows a chip inductor or a chip LC filter 63 or the like which is arranged to be mounted on a circuit board 64 of a surface-mount type, i.e., an SMT (surface mount technology) type. These parts have been used not only as ordinary circuit elements but also as noise-radiation preventing parts.
In a case where the known electronic parts shown in FIG. 6 are to be used as noise-radiation preventing parts, they are normally mounted on an electronic circuit board (a printed circuit board) 64 in the neighborhood of signal cable connection terminals.
Reflecting the technological trend mentioned above, image recording apparatuses such as video cameras have recently come to have their internal signal processing circuits digitized. Further, as a result of advancement of signal compressing technology, some of the currently marketed products have their video signal processing circuits arranged as digital circuits, such as digital signal processors (DSP), instead of conventional analog circuits. With a circuit which has conventionally been arranged in analog circuit thus digitized, the high frequency noise generated inside of a semiconductor device increases to endanger an electromagnetic environment around it as mentioned above. Then, the probability of violating the above-stated legal regulations of advanced countries also becomes high. In order to conform to the legal regulations, the noise radiation must be effectively prevented with some noise-radiation preventing part used for the manufacture of a product which never aggravates the electromagnetic environment around it.
In order to most effectively prevent the noise radiation, it is preferable to mount the noise-radiation preventing part in a position adjacent to a semiconductor device which includes the digital signal processing circuit, i.e., in a position where the noise-radiation preventing part can be connected directly to a connection pin or a connection lead wire protruding from the package of the semiconductor device.
However, since it is essential for designing a portable apparatus such as a video camera or the like to reduce its size and weight as well as to increase its functions, the electronic circuit board 64 mounted on the apparatus is also arranged in a reduced size. As a result, electronic parts are very densely arranged on the circuit board 64 and thus leaves a very little room for mounting the noise-radiation preventing part. Further, in these days, a semiconductor device which is composed of an IC, an LSI, etc., and has a high rate of integration includes therein numerous logic gates and transistors. A large number of connection pins, therefore, protrude from its package with very narrow spacing among them. Besides, these pins are very thin. It is, therefore, impossible to have a sufficient number of noise-radiation preventing parts mounted on the circuit board 64. Any attempt to mount these noise-radiation preventing parts on the circuit board 64 would necessitate a great increase in size of the circuit board 64 and thus would eventually increase the size and weight of the apparatus against its basic design policy.